1. Field of the Invention
The present invention relates to a system for determining the pressure of a pneumatic tire for use on a motor vehicle such as an automobile, and more particularly to a pneumatic tire pressure determining system for determining a reduction in the pneumatic tire pressure when the difference between the rotational speeds of road wheels of a motor vehicle does not fall within a predetermined range that depends on an operating condition of the motor vehicle.
2. Description of the Prior Art
Known pneumatic tire pressure determining systems are disclosed in Japanese patent publication No. 56-10202 and Japanese laid-open patent publication No. 63-305011, for example.
The pneumatic tire pressure determining system disclosed in Japanese laid-open patent publication No. 63-305011 detects the angular velocities of four road wheels, i.e., front left, front right, rear left, and rear right road wheels, of a motor vehicle, calculates the sums of the angular velocities of diagonal pairs of road wheels, and compares the angular velocity of each of the road wheels with the average value of the angular velocities of the four road wheels if the difference between the calculated sums falls within a predetermined range, thereby determining a reduction in the pneumatic pressure in any of the tires of the road wheels.
The disclosed pneumatic tire pressure determining system cannot determine the pneumatic tire pressures highly accurately and hence is not highly reliable because only the angular velocities of the road wheels are used as a reference value. More specifically, the pneumatic tire pressure determining system can determine a reduction in the pneumatic tire pressure of one road wheel, but fails to detect which one of the tires of the road wheels has suffered such a pneumatic tire pressure drop. Furthermore, in the event that the pneumatic tire pressures of two or three road wheels have dropped with a resultant reduction in the average angular velocity, the system cannot determine whether the pneumatic tire pressures have decreased or not.